Renal function, in nephrology, is an indication of the kidney's condition and its role in renal physiology.
Glomerular filtration rate (GFR) describes the flow rate of filtered fluid through the kidney.

Most doctors use the plasma concentrations of the waste substances of creatinine and urea (U), as well as electrolytes (E), to determine renal function.
These measures are adequate to determine whether a patient is suffering from kidney disease.

However, blood urea nitrogen (BUN) and creatinine will not be raised above the normal range until 60% of total kidney function is lost.
Hence, the more accurate Glomerular filtration rate or its approximation of the creatinine clearance is measured
whenever renal disease is suspected or careful dosing of nephrotoxic drugs is required. https://en.wikipedia.org/wiki/Renal_function

If the volume drained is modest ( 1-2 liters) and the patient stable and ambulatory,
attach the Foley catheter to a leg bag and discharge him, for followup (and probably, catheter removal) the next day.

If the volume drained is small (100-200ml), remove the catheter and search for alternate etiologies of the abdominal mass and urinary urgency.Urinary Retention

What causes urinary retention?
Urinary retention can result from

obstruction of the urethra
weakened bladder muscles

How is urinary retention treated?
A health care provider treats urinary retention with